Since conventional microfilm and microfiche deteriorate with time, recording information on a permanent medium as a surface relief pattern is a desireable means for storing information for archival purposes.
If a photoresist is used as the recording medium for a surface relief focused image hologram, the resulting phase hologram is recorded as an amplitude modulated, sinusoidal surface relief pattern on the surface of the photoresist. Since conventional photoresist materials are soft, it is often necessary to use techniques similar to those outlined in U.S. Pat. No. 3,565,978 to replicate the hologram in a hard material suitable for use as a master. This technique involves a metal plating procedure in which a metal film is built up on the resist surface and subsequently separated from the resist to form a metal master for replicating the hologram. Great care and precision are required to transfer the surface relief pattern to the metal master without distortion since the relief pattern dimensions are on the order of 1 micron.
Another technique for duplicating surface relief phase holograms involves etching the surface relief pattern recorded on a photoresist into a hard durable substrate material underlying the photoresist and the use of this substrate as a master for replicating the hologram in a suitable medium, e.g., a thermoplastic. U.S. Pat. No. 3,733,258 describes such a process using sputter-etching techniques. The substrate also may be etched by chemical etching techniques.
Conventional techniques described above for replicating surface relief focused image holograms are often uneconomical for reproducing small numbers, i.e., a few thousand copies of the holograms, due to the expense involved in producing the master. Therefore, it is highly desired to develop a quick, economical method for making masters for surface relief holograms.